by Thomas Guengerich
SOCORRO, N.M., June 12, 2008 -- New Mexico Tech Associate Professor of Geology Gary Axen has secured a National Science Foundation grant to study the mechanics of an important and poorly understood type of fault zone.
Axen and doctoral student Amy Luther, along with researchers from the University of New Mexico, will study two dormant fault zones in southern California. The researchers will examine the textures and chemical composition of fault rocks – rocks brought to the surface during basin-and-range rifting – to better understand the geological forces at work.
The Tech team will spend at least three years studying low-angle normal faults, which dip 20 to 30 degrees from horizontal and along which the rocks below the fault move up relative to those below. Classic theory about fault mechanics posits that high frictional resistance should not allow them to slip, but geologic evidence shows that they do.
“These faults bring up rocks representing the entire earthquake-generating part of the fault zone,” Axen said. “Instead of spending $20 million to drill a few kilometers to find these rocks, we have faults that are continuously exposed for 50 to 60 kilometers along the surface.”
In some cases, such as the San Andreas Fault, which appears to share the same mechanical peculiarity, rocks on either side move horizontally past each other. Axen’s current research project focuses on faults that have significant vertical movement – where geologic motions over millions of years bring deep fault rocks to the surface – some from as deep as 10 miles below Earth’s surface, Axen said.
The $320,000 NSF grant will support three seasons of work collecting rock samples and analyzing data. By studying the chemical composition of the fault rocks, Axen expects to learn about the geological forces that precipitate fault movement and earthquakes.
“It’s very common in science that there’s an elegant theory and then people find facts that fly in the face of the elegant theory. Then you have to revise or change the theory,” Axen said. “The elegant theory is that faults move by simple frictional slip with levels of frictional resistance like those measured in laboratory rock experiments.”
New Mexico Tech Earth and Environmental Science Department Chair Rick Aster complemented Axen on his new award.
“It’s indeed remarkable how complex fault zones are in the real Earth, and how much we still have to learn about how they create earthquakes as well as the mountains and valleys found in geologically active parts of our planet. This is cutting-edge research by very experienced field geologists that will make critical observations to help us figure this out,” Aster said.
Axen has spent nearly 20 years proving that low-angle normal faults exist and are important to the evolution of Earth’s continents. He theorizes that the laws of friction succumb to other processes at the geologic scale that are different than the laws observed in laboratory experiments.
“Existing simple theories of faults say they should not slip,” Axen said. “But they do slip. The resistance of these faults to slipping seems to be much lower than the friction measured in lab studies. Friction is a universal property and we understand friction at an engineering level. In geological terms, maybe simple friction is the wrong model.”
Axen first researched these faults as a doctoral student at Harvard University from 1987 to 1991. He continued his work as a professor at UCLA before coming to New Mexico Tech in 2005. His theories weren’t widely accepted at first.
“It took a long time to get over the inertia of disbelief,” Axen said. “There are still some people who don’t believe these low-angle faults exist.”
“When rocks slip past each other, they get crushed, chemically altered, and reconstituted into a new type of rock,” Axen said. “We’re doing a detailed set of electron microprobe and fluid-inclusion studies to understand the geologic evolution of these fault rocks.”
Axen and Luther will use optical microscopy and the New Mexico Tech Bureau of Geology electron microprobe to examine the textures and chemical composition of fault rocks collected from in the southern California fault zones.
“We’ll be able to understand what changes affected the original rocks to turn them into fault rock and what sorts of fluids were present in the fault zone,” Axen said. “Gathering data from these two examples should allow us to modify the theory to understand how these faults work.”
– NMT –